If the drag body is disposed in the middle of the fluid flow stream, vortices are alternately shed, i.e., separated, from both lateral edges of the drag body. The frequency of the vortex shedding, the so-called vortex separation frequency, is proportional to the velocity of flow, and thus--for wall-bounded fluid flows--proportional to the volumetric flow rate.
The measurement accuracy of the flow sensor is essentially dependent on the drag body, particularly on its shape, its dimensions, and its mounting point. Also dependent on these factors are that range of the Reynolds number Re in which the ratio of vortex separation frequency to volumetric flow rate is constant and which generally forms the measuring range of the vortex flow sensor, and the variation of the vortex separation frequency, which influences the measurement accuracy.
Prior art vortex flow sensors use simple basic shapes of the drag body, such as cylinder, rectangle, triangle, trapezoid, with trapezoidal drag bodies being frequently employed.
With such shapes of the drag body, however, a range of constant Strouhal number can only be achieved for Reynolds numbers Re.gtoreq.20,000, related to the diameter of the measuring tube; also, the variation of the Strouhal number is above 1%.
The Strouhal number S, as is well known, gives the relation between vortex separation frequency f and flow velocity v for a given width b of the drag body: S=fb/v.
with the above drag body shapes, the following values are reached, within optimum limits of measurement, for the lower limit Re.sub.min of the Reynolds number, and thus for the lower limit of the measuring range, as well as for the variation o of the vortex separation frequency f:
______________________________________ Shape of drag body Re.sub.min .sigma./% ______________________________________ Circle 40,000 4.0 Rectangle 25,000 3.0 Triangle, upstream side flat 21,000 1.5 Triangle, upstream side pointed 28,000 2.0 Trapezoid 25,000 1.0 Narrow trapezoid 20,000 2.0 Rounded trapezoid 25,000 3.0 T body 30,000 2.0 Trapezoid/trapezoid 20,000 2.5 ______________________________________
In DE-A-39 16 056, only a drawing of the cross section of a drag body is shown (cf. FIG. 4 therein) without further explanation,
said drag body being a combination body consisting of PA1 an upstream-side flat trapezoidal part PA1 of length l.sub.1, PA1 of base width b.sub.1 on the upstream side, and PA1 of base width b.sub.2 on the downstream side, and PA1 a wake part adjoining the trapezoidal part without a gap and having a cross section in the form of a triangle of length l.sub.2 and base width b.sub.3, PA1 with the following dimensioning equations holding: ##EQU1## PA1 which is permanently connected with the internal wall of a fluid-conducting measuring tube of inside diameter D, or fixed in a frame set in the measuring tube, at diametrically opposed points, and PA1 which is a combination body consisting of PA1 an upstream-side flat trapezoidal part PA1 of length l.sub.1, PA1 of base width b.sub.1 on the upstream side, and PA1 of base width b.sub.2 on the downstream side, and PA1 a wake part adjoining the trapezoidal part without a gap and having a cross section in the form of a triangle of length l.sub.2 and base width b.sub.3, PA1 with the following dimensioning equations holding: ##EQU2##
Also in common use are other combination bodies formed from the above basic shapes, which, if used in turbulent flow, however, give no appreciable increase in the range of constant Strouhal number or no reduction of the vortex separation frequency variation.